<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
  
  .item{
    position:fixed;
    width:100px;
    height:100px;
    left:0;
    top:0;
    background-color: #f00;
    
  }
  .box2{
    left:20px;
    top:20px;
    background-color: #0f0;
    z-index: -1;
  }
  .box3{
    left:40px;
    top:40px;
    background-color: #00f;
  }
  
  </style>
</head>
<body>
  <div class="item box1">1</div>
  <div class="container">
      <div class="item box2">2</div>
  </div>
 
  <div class="item box3">3</div>
  
</body>
</html>